Cholesterol has been known for many years to be a component of atherosclerotic plaques. Mounting evidence indicates diets high in cholesterol may increase the levels of cholesterol in the blood which, in turn, increase the risk of atherosclerotic disease and its attendant manifestations of heart attack, stroke and other tissue injuries resulting from atherosclerosis. Cholesterol absorbed from dietary sources is thought to increase the risk of atherosclerotic disease.
Other than avoidance or reduced consumption of high cholesterol foods, measures available without prescription to the general public to reduce the absorption of cholesterol from the diet have met with little success. Furthermore, in many cases, high serum cholesterol cannot be reduced by lowering dietary cholesterol. However, high cholesterol levels in serum can be lowered effectively by altering the intestinal metabolism of lipids. In recent years, it has become known that certain plant sterols and plant stanols such as .beta.-sitosterol (24-ethyl-5-cholestene-3.beta.-ol) and its hydrogenated form (.beta.-sitostanol (24-ethyl-5.alpha.-cholestane-3.beta.-ol) can help lower serum cholesterol by inhibiting cholesterol absorption in the digestive system. Plant stanols are the hydrogenated form of plant sterols. See, e.g., "Reduction of Serum Cholesterol With Sito Stanol-Ester Margarine in a Mildly Hypercholesterolemic Population", New England Journal of Medicine, Nov. 16, 1995, pp. 1308-1312.
The use of plant sterols, which are natural components of vegetable fats and oils, in food products is considered safe. Plant sterols themselves are not absorbed--or only absorbed in very small amounts--from the intestines. A decreased incidence of coronary disease is clearly associated with a decrease in serum cholesterol and, in particular, a decrease in LDL cholesterol. A high serum cholesterol level is one of the most significant indicators of risk of coronary disease. There are a variety of naturally occurring plant sterols which have been reported to have a cholesterol-reducing effect, although not all have equivalent action.
Although the mechanism by which plant sterols achieve the effect of lowering serum cholesterol has not been fully elucidated and not wishing to be limited by theory, it is believed that plant sterols interfere with cholesterol absorption by competition-type mechanisms. Cholesterol absorption appears to take place primarily in the proximal third of the small intestine. Cholesterol esters must be converted to their free hydroxyl form by the action of cholesterol esterases before they can be absorbed. The free cholesterol requires bile salts for solubilization and absorption. Bile salts form an aqueous dispersion of micelles in which the cholesterol is solubilized along with phospholipids and hydrolysis products of other dietary lipids. Micelles transport the cholesterol across the hydrophilic barrier (the unstirred water layer) to reach the surface of the intestinal mucosa. At the mucosa, it is thought that the cholesterol dissociates from the micelle and is transported into the mucosal cells by a process which has not yet been fully defined but may include passive exchange diffusion or by protein-mediated transport. Plant sterols could interfere with cholesterol absorption at either of, or at both of, the following: (a) competition with cholesterol for absorption into the bile-salt micelles or (b) competition with the transport mechanism into the mucosal cells. Whether plant sterols displace cholesterol from the micelles by mass action, or whether they compete with cholesterol for binding at the mucosal transport system, a soluble form of the plant sterol should be more effective than crystalline forms. In fact, it has been suggested that "the cholesterol-lowering effect of sitostanol may be increased when it is ingested in a soluble form." New England Journal of Medicine, Nov. 16, 1995, p.1308.
Conventionally, plant sterols have been incorporated into food products by melting a sterol or stanol, incorporating it into an oil phase, and blending the oil phase with other components to result in a plant sterol-containing food product. However, the plant sterols generally have high melting points (i.e., about 130-180.degree. C.) which can result in the crystallization of the plant sterols within the oil phase of such food products. Such crystallization results in food products with a gritty and unacceptable texture. This gritty texture is especially detectable when the oil/plant sterol phase is incorporated at high levels in the food product. Furthermore, the high melting points of such plant sterols makes it difficult to blend such plant sterols with an aqueous phase. Attempts have been made to solve this problem using chemical modification of the plant sterols. For example, esterification of plant sterols generally results in lowered melting temperatures. Thus, such plant sterol esters generally may be incorporated into food products more readily due to the lower melting points and can provide food products without significant gritty texture.
It would be desirable, therefore, to provide forms of plant sterols which can be incorporated into food products at relatively high levels without adversely affecting texture or other organoleptic properties. It would also be desirable to provide forms of plant sterols which can more easily be incorporated into food products. It would also be desirable to provide methods for preparing such plant sterols without chemical modification of the plant sterol itself. The present invention provides such plant sterols and methods for preparing them.